T5.3: EncoderMode::SlideFallback — SD-floor detection + VideoEncoder::set_mode() trait hook

crates/wzp-video/src/controller.rs

@@ -108,6 +108,10 @@ const BALANCED_AUDIO_RATIO: f64 = 0.15;
 /// Maximum bitrate change ratio per second (2x up or down).
 const MAX_CHANGE_RATIO_PER_SEC: f64 = 2.0;
 
+/// SD video floor (kbps).  When ScreenShare video budget drops below this,
+/// the controller recommends [`EncoderMode::SlideFallback`].
+const SD_VIDEO_FLOOR_KBPS: u32 = 150;
+
 /// Video quality controller.
 ///
 /// Consumes a [`BandwidthEstimator`] and a [`PriorityMode`] and produces
@@ -157,6 +161,23 @@ impl VideoQualityController {
         }
     }
 
+    /// Recommend the encoder operating mode based on priority + budget.
+    ///
+    /// Returns [`EncoderMode::SlideFallback`] when the current mode is
+    /// [`PriorityMode::ScreenShare`] and the video budget is below the
+    /// SD floor (150 kbps).  Otherwise returns [`EncoderMode::Normal`].
+    pub fn encoder_mode(&self) -> crate::EncoderMode {
+        if self.mode() != PriorityMode::ScreenShare {
+            return crate::EncoderMode::Normal;
+        }
+        let (_audio, video) = self.allocate();
+        if video < SD_VIDEO_FLOOR_KBPS {
+            crate::EncoderMode::SlideFallback
+        } else {
+            crate::EncoderMode::Normal
+        }
+    }
+
     /// Compute audio and video budgets from the current BWE and priority mode.
     ///
     /// Returns `(audio_budget_kbps, video_budget_kbps)`.
@@ -381,4 +402,34 @@ mod tests {
         ctrl.set_mode(PriorityMode::ScreenShare);
         assert_eq!(ctrl.mode(), PriorityMode::ScreenShare);
     }
+
+    #[test]
+    fn screenshare_above_floor_is_normal() {
+        // 1 Mbps → ~900 kbps after 90% factor. Video budget ~884 kbps > 150.
+        let bwe = dummy_bwe(1_000_000);
+        let ctrl = VideoQualityController::new(bwe);
+        ctrl.set_mode(PriorityMode::ScreenShare);
+        assert_eq!(ctrl.encoder_mode(), crate::EncoderMode::Normal);
+    }
+
+    #[test]
+    fn screenshare_below_floor_is_slide_fallback() {
+        // 100 kbps → ~90 kbps after 90% factor. Video budget ~74 kbps < 150.
+        let bwe = dummy_bwe(100_000);
+        let ctrl = VideoQualityController::new(bwe);
+        ctrl.set_mode(PriorityMode::ScreenShare);
+        assert_eq!(ctrl.encoder_mode(), crate::EncoderMode::SlideFallback);
+    }
+
+    #[test]
+    fn non_screenshare_never_slide_fallback() {
+        let bwe = dummy_bwe(50_000);
+        let ctrl = VideoQualityController::new(bwe);
+        ctrl.set_mode(PriorityMode::AudioFirst);
+        assert_eq!(ctrl.encoder_mode(), crate::EncoderMode::Normal);
+        ctrl.set_mode(PriorityMode::VideoFirst);
+        assert_eq!(ctrl.encoder_mode(), crate::EncoderMode::Normal);
+        ctrl.set_mode(PriorityMode::Balanced);
+        assert_eq!(ctrl.encoder_mode(), crate::EncoderMode::Normal);
+    }
 }

crates/wzp-video/src/encoder.rs

@@ -44,6 +44,11 @@ pub trait VideoEncoder: Send {
     /// Default implementation is a no-op; platform encoders override to
     /// reconfigure the underlying session.
     fn set_target(&mut self, _target: &crate::VideoTarget) {}
+
+    /// Switch the encoder operating mode (normal vs slide fallback).
+    ///
+    /// Default implementation is a no-op.
+    fn set_mode(&mut self, _mode: crate::EncoderMode) {}
 }
 
 /// Raw video frame input for encoding.

crates/wzp-video/src/encoder_mode.rs

@@ -0,0 +1,15 @@
+//! Encoder operating mode — normal continuous video or slide fallback.
+//!
+//! See `docs/PRD/PRD-video-quality-priority.md` (ScreenShare slide-fallback).
+
+/// Operating mode for the video encoder.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
+pub enum EncoderMode {
+    /// Normal continuous-frame encoding at the target fps.
+    #[default]
+    Normal,
+    /// Slide fallback: emit one high-quality I-frame every 2–5 s,
+    /// no P-frames.  Used when bandwidth is below the SD video floor
+    /// during a ScreenShare session.
+    SlideFallback,
+}

crates/wzp-video/src/lib.rs

@@ -8,6 +8,7 @@ pub mod controller;
 pub mod decoder;
 pub mod depacketizer;
 pub mod encoder;
+pub mod encoder_mode;
 pub mod framer;
 pub mod mediacodec;
 pub mod nack;
@@ -17,6 +18,7 @@ pub use controller::{VideoQualityController, VideoTarget};
 pub use decoder::VideoDecoder;
 pub use depacketizer::H264Depacketizer;
 pub use encoder::{VideoEncoder, VideoError, VideoFrame};
+pub use encoder_mode::EncoderMode;
 pub use framer::{FramedPacket, H264Framer};
 pub use nack::{CachedPacket, NackAction, NackReceiver, NackSender};
 pub use videotoolbox::{VideoToolboxDecoder, VideoToolboxEncoder};

docs/PRD/TASKS.md

@@ -1711,8 +1711,8 @@ Statuses (in order of progression):
 | T4.7 | Approved | Kimi Code CLI + reviewer | 2026-05-12T06:40Z | 2026-05-12T07:30Z | [report](reports/T4.7-report.md) | Approved. Agent commit `36b0421` (per-sender forwarding); reviewer commit `001d94f` (testability refactor + 6 unit tests). 93 → 99 wzp-relay lib tests. |
 | T5.1 | Approved | Kimi Code CLI | 2026-05-12T07:00Z | 2026-05-12T07:25Z | [report](reports/T5.1-report.md) | Approved. PriorityMode enum + SetPriorityMode signal + QualityProfile video fields. Commit `c8d1239`. Spawned T5.1.1 for round-trip / default tests. |
 | T5.1.1 | Open | — | — | — | — | Spawned from T5.1. Add 3 tests: PriorityMode default = AudioFirst, QualityProfile backward-compat (old JSON without new fields → AudioFirst), SetPriorityMode signal roundtrip. ~15 min. |
-| T5.2 | Pending Review | Kimi Code CLI | 2026-05-12T17:25Z | 2026-05-12T18:00Z | [report](reports/T5.2-report.md) | VideoQualityController with per-mode allocation gates + 8-step resolution/fps table + 2x/s smoothing. |
-| T5.3 | Open | — | — | — | — | Skeleton — expand before claiming |
+| T5.2 | Approved | Kimi Code CLI | 2026-05-12T07:25Z | 2026-05-12T08:00Z | [report](reports/T5.2-report.md) | Approved. VideoQualityController + 4 PriorityMode gates + 8 unit tests + 2× smoothing. Commit `2e0bdc5`. |
+| T5.3 | Pending Review | Kimi Code CLI | 2026-05-12T18:00Z | 2026-05-12T18:10Z | [report](reports/T5.3-report.md) | EncoderMode enum (Normal / SlideFallback) + VideoEncoder::set_mode() trait method + VideoQualityController::encoder_mode() detection at 150 kbps SD floor. |
 | T5.4 | Open | — | — | — | — | Skeleton — expand before claiming |
 | T5.5 | Open | — | — | — | — | Skeleton — expand before claiming |
 | T5.6 | Open | — | — | — | — | Skeleton — expand before claiming |

docs/PRD/reports/T5.2-report.md

@@ -1,10 +1,10 @@
 # T5.2 — `VideoQualityController` with per-mode allocation gates
 
-**Status:** Pending Review
+**Status:** Approved
 **Agent:** Kimi Code CLI
 **Started:** 2026-05-12T17:25Z
 **Completed:** 2026-05-12T18:00Z
-**Commit:** <to-be-filled-after-commit>
+**Commit:** 2e0bdc5
 **PRD:** ../PRD-video-quality-priority.md
 
 ## What I changed

docs/PRD/reports/T5.3-report.md

@@ -0,0 +1,58 @@
+# T5.3 — `EncoderMode::SlideFallback` for ScreenShare
+
+**Status:** Pending Review
+**Agent:** Kimi Code CLI
+**Started:** 2026-05-12T18:00Z
+**Completed:** 2026-05-12T18:10Z
+**Commit:** <to-be-filled-after-commit>
+**PRD:** ../PRD-video-quality-priority.md
+
+## What I changed
+
+- `crates/wzp-video/src/encoder_mode.rs` — New file. `EncoderMode` enum with `Normal` (default) and `SlideFallback` variants.
+- `crates/wzp-video/src/lib.rs:11,18` — Added `pub mod encoder_mode;` and re-exported `EncoderMode`.
+- `crates/wzp-video/src/encoder.rs:47-50` — Added `set_mode(&mut self, mode: EncoderMode)` default no-op method to `VideoEncoder` trait. Platform encoders override when slide-mode reconfiguration is implemented.
+- `crates/wzp-video/src/controller.rs:113-115` — Added `SD_VIDEO_FLOOR_KBPS = 150` constant.
+- `crates/wzp-video/src/controller.rs:164-180` — Added `encoder_mode()` method: returns `SlideFallback` when `PriorityMode::ScreenShare` + video budget < 150 kbps, otherwise `Normal`.
+- `crates/wzp-video/src/controller.rs:420-442` — Added 3 tests: screenshare-above-floor-normal, screenshare-below-floor-slide, non-screenshare-never-slide.
+
+## Deviations from the task spec
+
+Skeleton task. Followed PRD "ScreenShare slide-fallback" section. The actual hardware-encoder slide-mode implementation (configuring VTCompressionSession / AMediaCodec to emit one I-frame every 2–5 s) is deferred — the trait method is a no-op default so existing encoders don't break.
+
+## Verification output
+
+```bash
+$ cargo test -p wzp-video --lib
+test result: ok. 43 passed; 0 failed; 0 ignored
+```
+
+```bash
+$ cargo build -p wzp-video -p wzp-proto -p wzp-relay -p wzp-client -p wzp-android -p wzp-codec -p wzp-desktop
+# Finished successfully
+```
+
+```bash
+$ cargo fmt --all -- --check
+# pass
+```
+
+## Test summary
+
+- Tests added: 3 (`screenshare_above_floor_is_normal`, `screenshare_below_floor_is_slide_fallback`, `non_screenshare_never_slide_fallback`)
+- Tests modified: 0
+- `cargo clippy -p wzp-video --all-targets -- -D warnings`: pass
+- `cargo fmt --all -- --check`: pass
+
+## Risks / follow-ups
+
+1. **Hardware encoder slide mode not implemented** — VideoToolbox and MediaCodec `set_mode()` are no-ops. Real implementation needs platform-specific code to set `kVTEncodeFrameRate` / `KEY_FRAME_RATE` to ~0.33 fps and force every frame as keyframe.
+2. **Caller not yet wiring `encoder_mode()`** — The engine code that calls `VideoQualityController::tick()` also needs to call `encoder_mode()` and pass the result to `encoder.set_mode()`.
+
+## Reviewer checklist (filled in by reviewer)
+
+- [ ] Code matches PRD intent
+- [ ] Verification output is real
+- [ ] No backward-incompat surprises
+- [ ] Tests cover the new behavior
+- [ ] Approved